Answer :
Final answer:
Poison gas was the weapon used by Germans in WWI and was later outlawed by the 1925 Geneva Protocol due to its grave effects on human health.
Explanation:
The weapon used by the Germans in WWI that was later declared to be illegal by the Geneva Convention is poison gas. German troops first used chemical weapons like chlorine gas against Russian units in 1914. Other gases such as mustard gas and phosgene were introduced later in the war, with devastating effects on the health and wellbeing of soldiers. These gases could cause severe burns, respiratory problems, and even death. Due to the horrific nature of chemical weapons, the 1925 Geneva Protocol specifically banned all forms of chemical warfare.
